The overall objective of our project is to characterize the properties of measles virus which are required for virus persistence by using various in vitro and in vivo techniques. It is also our intent to determine what, if any, relationship exists between factors regulating virus persistence and neurovirulence. Goals for the current year included: 1) the further analysis of a hemagglutination defective variant of measles virus (SSPE-BIKEN), 2). evaluation of the role that measles virus variants and defective interfering particles play in the maintenance of persistent infections, 3). the characterization of measles virus polypeptides synthesized in a persistently infected hamster cell line (S cells), and 4) preliminary studies on the role of cell factors in virus persistence.